This type of loudspeaker enclosure was proposed in October 1965 by Dr A.R. Bailey and A.H. Radford in Wireless World magazine.
The article postulated that energy from the rear of a driver unit could be essentially absorbed, without damping the cone's motion or superimposing
internal reflections and resonance, so Bailey and Radford reasoned that the rear wave could be channelled down a long pipe.
If the acoustic energy was absorbed, it would not be available to excite resonances. A pipe of sufficient length could be tapered, and stuffed
so that the enegy loss was almost complete, minimizing output from the open end. No broad consensus on the ideal taper (expanding, uniform cross-section,
or contacting) has been established.
Unlike vented speaker systems, transmission lines are designed to absorb most of the energy radiated from the driver's rear, and if appropriately damped,
the line does not form a tuned system. Closed type transmission lines typically have negligible acoustic output from the enclosure except from the driver.
Open ended lines exploit the low-pass filter effect of the line, and the resultant low bass energy emerges to reinforce the output from the driver at
low frequencies. Well designed transmission line enclosures have smooth impedance curves, possibly from a lack of frequency-specific resonances,
but a low efficiency.
Have also a look at my second Transmission Line Calculator